1. Field of the Invention
The present invention is a probe for an electro-optical sampling oscilloscope that couples an electrical field generated by a measured signal and an electro-optic crystal, inputs into this electro-optic crystal a pulse generated based on a timing signal from a timing generation circuit, and observes the waveform of the measured light by the state of the polarization of the input light pulse, and relates in particular to an electro-optic sampling oscilloscope probe having an improved optical system.
This application is based on Japanese Patent Application, No. Hei 10-148033 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
It is possible to couple an electrical field generated by a measured signal with an electro-optic crystal, input a laser beam into this electro-optic crystal, and observe the waveform of the measured signal by the state of the polarization of the laser beam. It is possible pulse the laser beam, and observe with an extremely high time resolution when sampling the measured signal. The electro-optic sampling oscilloscope uses an electro-optic probe exploiting this phenomenon.
When this electro-optic sampling oscilloscope (hereinbelow, referred to as an "EOS oscilloscope") is compared to a conventional sampling oscilloscope using an electrical probe, the following characteristics have received much attention:
1. It is ease to observe the signal because a ground wire is unnecessary.
2. Because the metal pins at the end of the electro-optic probe are not connected to the circuit system, it is possible to realize high input impedance, and as a result of this, there is almost no degradation of the state of the measured point.
3. By using an optical pulse, broadband measurement up to the GHz order is possible.
The structure of a probe for an EOS oscilloscope in the conventional technology will be explained using FIG. 4. In FIG. 4, reference numeral 1 is a probe head made from an insulating body, and a metal pin 1a is inserted into its center. Reference numeral 2 is an electro-optic element, in which the reflecting film 2a is provided on the side facing the end surface of the metal pin 1a so as to abut the metal pin 1a. Reference numeral 4 is a half-wave plate, and reference numeral 5 is a quarter-wave plate. Reference numerals 6 and 9 are polarization beam splitters. Reference numeral 7 is a half-wave plate, and reference numeral 8 is a Faraday rotator. Reference numeral 10 is a collimator lens, and reference numeral 11 is a laser diode. Reference numerals 12 and 14 are condenser lenses. Reference numerals 13 and 15 are photodiodes.
In addition, the two polarization beam splitters 6 and 9, the half-wave plate 7, and the Faraday rotator 8 comprise an isolator 16 that transmits the light emitted from the laser diode 11, in order to split the light reflected by the reflecting film 2a.
Next, referring to FIG. 4, the path of the laser beam emitted form the laser diode 11 is explained. In FIG. 4, reference letter `A` denotes the path of the laser beam.
First, the laser beam emitted from the laser diode 11 is converted by the collimator lens 10 into a parallel beam that travels straight through the polarization beam splitter 9, the Faraday rotator 8, the half-wave plate 7, and the polarization beam splitters 6, and then transits the quarter-wave plate 5 and the half-wave plate 4, and is incident on the electro-optic element 2. The incident light is reflected by the reflecting film 2a formed on the end surface of the electro-optic element 2 on the side facing the metallic pin 1a.
The reflected laser beam transits the half-wave plate 4 and the quarter-wave plate 5, one part of the laser beam is reflected by the polarized light beam splitter 6, condensed by the condenser lens 12, and incident on the photodiode 13. The laser beam that has transited the polarized light beam splitter 6, is reflected by the polarization beam splitter 9, condensed by the condenser lens 14, and incident on the photodiode 15.
Moreover, the half-wave plate 4 and the quarter-wave plate 5 adjust the strength of the laser beam incident on the photodiode 13 and the photodiode 15 so as to be uniform.
Next, using the EOS oscilloscope probe shown in FIG. 4, the procedure for measuring the measured signal is explained.
When the metallic pin 1a is placed in contact with the measurement point, due to the voltage applied to the metallic pin 1a, at the electro-optic element 2 this electrical field is propagated to the electro-optic element 2, and the phenomenon of the altering of the refractive index due to the Pockels effect occurs. Thereby, the laser beam emitted form the laser diode 11 is incident on the electro-optic element 2, and when the laser beam is propagated along the electro-optic element 2, the polarization state of the beam changes. Additionally, the laser beam having this changed polarization state is reflected by the reflecting film 2a, is condensed and incident of the photodiode 13 and the photodiode 15, and converted into an electrical signal.
Along with the change in the voltage at the measurement point, the change in the state of polarization by the electro-optic element 2 becomes the output difference between the photodiode 13 and the photodiode 15, and by detecting this output difference, it is possible to observe the electrical signal applied to the metallic pin 1a.
In this connection, in an EOS oscilloscope, because an effect wherein the electric field propagated in the metallic pin 1a causes a change in the polarization state of the beam propagating through the electro-optic element 2, when attempting to convert efficiently the change in electrical field due to the measuring signal to the change in the polarized beam, it is preferable that the beam propagating in the electrooptic element 2 be narrower than the diameter of the metallic pin 1a.
In addition, the electro-optic element 2 is a structure that reflects in incident laser beam by a reflecting film 2a, but in order to make the incidence of the reflected beam on the two photodiodes 13 and 15 efficient, it is necessary to make the incident and reflected axes of the parallel beam coincide. When the optical axes of the incident and the reflected beam do not coincide, as shown in FIG. 4, the laser beam follows the light path shown by this broken lines (reference letter B in FIG. 4), and the laser beam is not incident on the photodiodes 13 and 15. Thus, the reflecting film 2a must be disposed orthogonally to the optical axis of the parallel beam.
However, in order to dispose orthogonally the incident and reflecting axes of the parallel beam and the reflecting film 2a, there is the problem that a high precision aligning technology and a large amount of alignment time are necessary.
In addition, when replacing the probe head 1 that anchors the electro-optic element 2, it is difficult to anchor the reflecting film 2a on the end surface of the electro-optic element 2 of the probe head 1 in the same position as that prior to replacement, and thus there is the problem that an alignment operation for realigning the optical axes is necessary.
In addition, the plane of polarization of the polarized light beam splitter 6 and the polarized light beam splitter 9, which split the reflected light, must coincide so that the photodiode 13 and the photodiode 15 can be disposed on the same plane. Due to this, in order to cause the polarized light surface that is rotated 45.degree. by the electro-optic element 2 to the original orientation, the half-wave plate 7 is provided, and thus number of structural components becomes large, the alignment points become numerous, and the unnecessary internal reflected light increases.